Usually, soft water is recommended as a minimum for the cleaning of surgical instruments. Not for the final rinse where deionised or demineralised water is the minimum standard. But what is soft water? is it the same as softened water?
In general, the water hardness refers to the level of Calcium Carbonate (CaCO3) content. There is not an exact value up to which level of CaCO3 content you can consider soft water but let´s say that there is a kind of consensus that less than 50 ppm of CaCO3 (50 mg/l CaCO3) content could be considered soft water [1]. At least this figure is recommended for long time [1] and it can be found in most technical specifications of Washer Disinfectors as well as on instruments manufacturers recommendations, as already said, only for the cleaning phase but not for the rinse/final rinse.
So, soft water is water with a maximum content of 50 ppm of CaCO3 (internationally also 0,5 mmol/l CaCO3). This water can be found naturally in some places or, it can be obtained after a so-called softening treatment. This kind of treatment is exchanging the Calcium and Magnesium cations with sodium cations in an ion-exchange-resin. The amount of sodium ions directly depends on the amount of Calcium and Magnesium ions coming from the incoming water, so the total salt content is nearly the same as the “raw” water (in practice, the total salt content is slightly larger since one Ca2+ cation will be exchanged against 2 Na+ cations with a total slightly higher mass). This means that both types of waters (naturally soft and softened) will have the same hardness, but the softened water from the softening process of hard water might have more minerals. And the hardest the incoming water is, the higher the total content of salts will be on the softened water. So here we have one point to say that naturally soft water is not necessarily the same as softened water.
A second important point is that the softened water may also have a higher content of chlorides coming from the self-regeneration of the water softener resin if not regenerated as required, or from malfunction of automated regeneration, or from a “breakdown”. As chlorides might not be considered as a “parameter” of soft (or softened) water, there could be again a situation of softened water with the same hardness as the natural soft water, but with higher chloride content. Both being soft water. And, as widely known, chlorides are a big harm for stainless steel and could lead to corrosion either on the machine components as well as on the surgical instruments and therefore it must be avoided [1].
In conclusion, from a definition perspective both soft and softened water (below 50 ppm CaCO3) are the same, however softened water could contain a higher salt content and may contain a higher chloride content. Therefore, for surgical instruments cleaning it should be advised to keep an exhaust control of the chloride content of the water and/or use demineralised water not only for the rinse/final rinse but also for the cleaning and even for the precleaning.
[1] Workgroup Instrument Reprocessing: Reprocessing of Instruments to Retain Value; 11th edition 2017; www.a-k-i.org
About the author
Matías Pilasi is member of AKI since October 2019 as ambassador for Latin America. He is specialized in the field of Hospital Hygiene, Cleaning, Disinfection, Packing and Sterilization as well as Process Validation in the Reprocessing of Medical Devices.
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